1. Field of the Invention
This invention relates generally to the field of projection display systems. More particularly, this invention relates to a projection display system where a modulated laser beam is scanned across a surface by a system of rotating and non-rotating mirrors and where areas of the surface illuminated by the laser form pixels of a video image.
2. Description of Related Art
The present invention is an improvement to a projection display device disclosed in two co-pending applications, 7229/53235, U.S. patent application No. 08/854,872 and 7229/53243, U.S. patent application No. 08/889,405.
In 7229/53235, U.S. patent application No. 08/854,872, a beam from a laser is scanned across a viewing surface by vertical and horizontal scanning rotating mirrors. A video signal modulates the intensity of the beam. The beam is first reflected from the vertical scanning mirror to position the beam for scanning along a particular horizontal scan line. The beam is then reflected from the horizontal scanning mirror to scan the beam across the viewing surface.
Such a system is an improvement over known projection displays because the resolution and aspect ratio of the scanned image can be dynamically adjusted. Also, by using intense laser light sources, images can be projected on large viewing surfaces to create, for example, outdoor displays.
That system suffers from the problem that the rotation rate for the horizontal scanning mirror must be very high in order to properly display a video signal. For example, to display an NTSC video signal with 330 horizontal scan lines per field at a rate of 30 fields per second, a sixteen-sided horizontal scanning mirror must rotate over 600 times per second.
High rotation rates make this system susceptible to wear and vibration. Also, the radial dimension of the horizontal mirror must be small, so that the centripetal forces on the mirror do not distort its reflective surfaces. Use of small mirrors requires precise alignment of the optical components. Construction and maintenance of this precise alignment increases the cost and reduces the reliability of the system.
Co-pending application, 7229/53243, U.S. patent application No. 08/889,405, provides a solution by allowing the rotation rate of the horizontal scanning mirror to be reduced. A group of fixed mirrors are positioned so that each face of the horizontal scanning mirror sweeps the beam across the viewing surface twice. As a result, the rotational rate of the horizontal scanning mirror is reduced by a factor of one half.
Both of the systems described above suffer from the problem that the mechanical tolerances of the rotating mirrors are critical to properly scan the beam across the viewing surface. Also, the edges of each reflective face of the rotating mirrors are susceptible to distortion due to manufacturing defects. The presence of these defects reduces the usable reflective area, thus diminishing the maximum viewing area scanned by the beam on the viewing surface.
Further, alignment of the rotating and fixed mirrors in these systems is problematic both during manufacturing and when the product is in use. Realignment is necessary as parts wear and as dimensions change due to thermal expansion, as well as to correct misalignment caused by vibration. The need for precise alignment requires a skilled technician and expensive measurement equipment. This increases both the manufacturing cost and the cost of ownership of these systems.